Once-through vapor generator



Nov. 2, 1965 T. s. SPRAGUE 3,215,125

ONCE-THROUGH VAPOR GENERATOR Filed Dec. 19, 1960 JSTEAM FLow METER 40 T I l 34 TURBINE 1O OR|F|cE- BLow-DowN 4 l VALVE 3 ELEMENT HEAT FEED' WATER SouRcE CONTROL CONDENSER 14 2 VAPOR GENERATOR l FEEDWATER FEED WATER FLOW METER CONTROL DEAERATOR 18 41 VALVE INVENTOR.

Theodore S. Sprague ATTORNE;

United States Patent 3,215,126 ONCE-THROUGH VAPOR GENERATOR Theodore S. Sprague, Hudson, Ohio, assignor to The Babcocl; & Wilcox Company, New York, N.Y., a corporation of New Jersey Filed Dec. 19, 1960, Ser. No. 76,596 3 Claims. (Cl. 122-451) This invention relates in general to a once-through vapor generating unit and more particularly it relates to a once-through vapor generating unit supplying saturated vapor which has been generated in a number of parallel tubes.

Once-through type vapor generating units usually consist of a number of parallel tube circuits heated by a common source in which a vaporizable liquid is distributed to the inlets of the tubes with the corresponding portions of liquid being vaporized as they progress through the heated tubes. It is common in units of this type that certain of the parallel tubes will receive greater quantities of heat than the remaining portions thus causing a larger quantity of vapor to be generated therein with a consequential unbalancing of flow rates between the parallel units. Thus, it is often the case that in some tubes the vapor is super-heated while in others it is just a mixture of saturated steam and water. Conditions such as these are variously referred to as unbalances or upsets. The condition sometimes can become aggravated and lead to tube burnout but, in any event, the difference in tube metal temperature between parallel tubes has detrimental effects over a long period thus tending to shorten their life. Thus, it can be said that prior art once-through vapor generating units are subject to unbalances which, if they could be avoided, would improve such units.

Once-through vapor generating units are also characterized by having a small quantity of liquid in the system at one time and when contrasted with drum type units do not have as good ability to meet sudden increases in steam rate demands. Drum boilers on the other hand, by having a large quantity of saturated liquid in the drum may meet sudden demands by allowing portions of the liquid to flash upon the sudden reduction in pressure caused by the demand. Thus, the peaks of the demand may be met due to the thermal inertia of the unit. This is sometimes referred to as thermal fly Wheel effect. Thus, it can also be said that once-through boilers could be improved if there can be provided means for meeting sudden increases in load.

The lack of a rapid response characteristic in oncethrough vapor generators has inhibited their application to marine power plants where sudden demands in loads are basic characteristics and to nuclear reactors which are characterized by their ability to respond to load swings by rapid changes in heat release.

This invention provides the best features of a drum type unit while maintaining the obvious advantages of a once-through vapor generator having a multiplicity of parallel long small bore tubes arranged to generate vapor from a heat source where said tubes discharge vapor in parallel to a vapor-liquid drum. The drum has a liquid level separating a body of liquid from an upper vapor space. A feed water control responsive to the liquid level in the drum maintains a flow of liquid through the tubes substantially equal to the vapor demand on the unit. The body of liquid in the drum provides a reservoir for sudden vapor demands and the drum also serves as a mixing device to desuperheat steam issuing from unbalanced circuits such that the steam issuing from the drum is all saturated.

The invention also provides for a means to pass small quantities of the vapor directly to the liquid space of the drum to assure a uniform temperature throughout.

Further, the invention provides a downcomer from the drum to the inlets of the parallel tube circuits having a valve therein which permits natural circulation of the fluid from the drum to the inlets of the tubes under certain low load conditions.

The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this specification. For a better understanding of the invention, its operating advantages and specific objects attained by its use, reference should be had to the accompanying drawing and descriptive matter in which a specific embodiment of the invention is illustrated and described.

In the drawing there is schematically shown a steam cycle arrangement utilizing a once-through boiler of the invention in which there is a once-through boiler 2 having a number of parallel tubes 4 heated by a heat source 8 and discharging into a steam drum 6. A steam turbine 10 is arranged remote from the boiler to receive vapor from the drum 6 via steam line 12. A condenser 14 receives the low pressure steam discharged from the turbine, condenses the vapor and delivers condensate via line 16 under the impulse of a condensate pump 18 to a deaerator 26. The deaerator is arranged to receive low pressure steam bled interstage from a portion of the turbine via line 22 and to deliver deaerated condensate to the suction of feed pump 24 for ultimate delivery to the tubes 4 of the boiler.

The tubes 4 are schematically shown as a single circuit, but in actuality would be represented by a multiplicity of very long small bore tubes arranged in parallel and receiving vaporizable liquid distributed to them from an inlet header 26. The tubes 4 would be exposed to heat from a heat source such as fossile fuel fired burners or heated fluid from a nuclear reactor system. The outlets of the tubes 4 discharge into a collection header 28 for further passage via lines 32 and 34 to the steam drum 6. Alternately, tubular circuits 4 can be arranged to discharge directly into the drum 6, in which case at least one circuit should be discharged below the water level in order to insure saturated temperature of the stored water.

Within the drum there is a body of liquid 36 below an upper vapor space 38 to thus provide a body of liquid maintained at saturated temperature by a small quantity of the heated fluid which is bled into the liquid portion through line 32.

The portion of fluid entering from line 34 would probably be a mixture of superheated and saturated steam with some unvaporized liquid which, upon discharge into the drum 38, will come to an equilibrium at saturated steam temperature. There may be arranged steam separating or scrubbing units (not shown) within the drum to perform the necessary function of preventing liquid from passing into the exit steam line 12. Line 12 passes to a point of use and contains a vapor flow meter 40.

An emergency liquid line 42 is arranged in communication with the liquid space in the bottom of the drum 6 and has branch portions 42A and 4213. 42A is a valve controlled blow down line and communicates with the deaerator 20. Branch 42B contains a check valve 44 and communicates with the inlet header 26 to parallel tubes 4 of the vapor generator. The check valve is arranged such that when the pressure on the tube inlet side is greater than on the drum side of the valve, it will remain closed thus permitting flow only in the direction from the drum to the inlet header.

A control 46 is arranged to receive a control signal responsive to a water level indication from a device 48 located on the drum 6 and a signal proportional to the steam flow from the meter 40 and from the feed water 3. flow meter 41. The control then passes an operating signal to a feed water control valve 50 located in the feed line from the discharge of the feed pump 24. The control 46 may be of any well known type now used in the vapor generator art.

With a view to the apparatus already described and to a better understanding of the invention, a typical operating cycle will now be described. High pressure liquid will be supplied from the deaerator by the feed pump 24 through the feed control valve 50 into the inlet header 26. There it will be distributed to the multiplicity of parallel tubes 4 wherein it will be heated due to its heat transfer relationship with the heat source. The heated fluid will be collected in outlet header 28 and distributed by lines 32 and 34 to the drum 6. The quantity of feed liquid will be predeterminedly set by the control 46 to assure that the rate of feed to the boiler circuits as measured by the meter 41 is just equal to the vapor flow rate received from the flow meter 40. The liquid level indication from the device 48 will bias the control 46 to provide either an overflow or underflow of liquid to accomplish the control of liquid level to a predetermined position within the drum 6. Thus, in the ordinary circumstances, the liquid of the volume 36 will not enter into the evaporation of the unit, but is merely there as a reservoir in the event of a sudden load demand, the water 36 being maintained heated and saturated by the small quantity of flow passing through the orificed line 32.

As aforesaid, it is inherent in once-through boilers that some circuit unbalance will occur thus causing some of the heated fluid flowing in the parallel tubes 4 to be superheated while others contain some liquid and saturated steam. The combination of the mixing header 28 and the drum 6 will cause mixing of these various heated fluids with the resultant equilibrium. Thus saturated vapor passes through the outlet line 12 to the turbine. In accomplishing this, the superheated vapor will be desuperheated and cause the evaporation of the remaining liquid portions.

It is, of course, important to the arrangement that the flow control valve maintain a flow substantially equal to the steam demand on the vapor generating unit at all times with only small biasing adjustments to accomplish the water level control in the drum. Thus, there is provided a once-through boiler having all of its advantages plus the added advantage of the thermal fly wheel efl ect embodied in the drum. Moreover, the drum additionally provides an exceptionally good mixing device for equalizing unbalance in the once-through boiler and assures that only saturated vapor will flow to the point of use.

Those skilled in the art will, of course, recognize that the point of use may be as illustrated in the drawing, but may also be a superheater which is also in contact with the heating source.

The branch line 42 serves a very useful purpose in emergencies where, when the drum is elevated above the outlet of the tubes 4, the branch line 42B may serve as a downcomer for the natural recirculation of a vaporizable liquid from the drum liquid space 36 to the inlet of the tubes 4. A situation like this would occur should the feed pump 24 stop momentarily or its outlet pressure fall below or substantially equal to the pressure head built up between the drum and the inlet header 26. Thus, for instance, in a nuclear reactor system there could be provided an emergency arrangement for the natural circulation of a vaporizable liquid to remove the decay heat from the core without requiring the services of the feed pump. This added feature would make this boiler unit very attractive for shipboard use in connection with a reactor system.

Branch line 42A provides a blow down arrangement such that when the solid content in the water 36 becomes extremely high, it can be periodically blown down to reduce that solid content in accordance with the ordinary practice connected with natural circulation drum boilers.

Applicant has discovered that a unit of this type would require a drum water surface area substantially less by at least a factor of two in comparison to a natural circulation drum type boiler. This, of course, is advantageous because it reduces the Weight of the unit plus fabrication expenses.

It is important to note that the drum 6 has as its primary function a reservoir of saturated liquid which may be flashed upon a sudden load demand and the eflicicnt equalization of heat content of the variously heated fluids issuing from the multiple parallel tubes 4. Whatever steam separation function it may serve is extremely minor due to the fact that the steam flow through the unit is only equal to the feed rate. Thus, none of the circuits would contain any sizeable quantity of unvaporized liquids.

Certain prior art once-through vapor generating units have considered the use of separating drums, but in such instances these units have usually required a continuous blow down or spill over from the drums, the drums serving the purpose of separating unvaporized liquid from the remaining vapor, such liquid being intended to carry the principal part of the dissolved solids from the system. It should be noted that in these units, the usual five (5) percent or so of spill over is suddenly reduced in pressure and passed to a low pressure system resulting in the thermodynamic degradation of the cycle. In contrast, the present invention has no such loss associated with it.

In these prior art vapor generating units having separators there was usually only one separator per circuit with the total unit containing a number of separate separators in parallel. Thus, when the unit suffered from unbalance of heating there was no device available to equalize the unbalance as provided in the present invention.

While in accordance with the provisions of the statutes, we have illustrated and described herein a special form of the invention now known to me, those skilled in the art will understand that changes may be made in the form of the apparatus disclosed without departing from the spirit of the invention covered by my claims, and that certain features of the invention may sometimes be used to advantage without a corresponding use of the other features.

What is claimed is:

1. In a once-through vapor generating unit for supplying vapor to a point of use, a thermal energy source, a multiplicity of long small bore tubes arranged in parallel flow relation to generate vapor from the heat of said source, means supplying a vaporizable liquid to the inlets of said tubes, a drum arranged to contain a body of liquid below an upper vapor space and arranged to receive the heated fluid from said tubes, means for continuously passing a small quantity of the heated fluid discharging from said tubes directly to the liquid space of said drum and for passing the remainder of said heated fluid to the vapor space of said drum, and a control responsive to the liquid level variations in said drum to maintain a rate of flow of said fluid through said tubes substantially equal to the vapor demand on said unit.

2. In a once-through vapor generating unit for supplying vapor to a point of use, a thermal energy source, a multiplicity of long small bore tubes arranged in parallel flow relation to generate vapor from the heat of said source, means supplying a vaporizable liquid to the inlets of said tubes, a drum arranged to contain a body of liquid below an upper vapor space and arranged to receive the heated fluid from said tubes, a control responsive to the liquid level variations in said drum to maintain a rate of flow of said fluid through said tubes substantially equal to the vapor demand on said unit, and a downcomer from the liquid space of said drum to the inlets of said tubes containing a normally closed valve adapted to permit flow from said drum to said tubes when the pressure on the drum side of said valve exceeds the pressure on the tube inlet side of said valve.

3. In a once-through vapor generating unit for supplying saturated vapor to a point of use, a thermal energy source, a multiplicity of long small bore tubes arranged in parallel flow relation to generate vapor from the heat of said source, means supplying a vaporizable liquid to the inlets of said tubes, a drum arranged to contain a body of liquid below an upper vapor space and arranged to receive the heated fluid from said tubes, means for continuously passing a small quantity of the heated fluid discharging from said tubes directly to the liquid space of said drum, a control responsive to the liquid level variations in said drum to maintain a rate of flow of said fluid through said tubes substantially equal to the vapor demand on said unit, and a downcomer from the liquid space of said drum to the inlets of said tubes containing a normally closed valve adapted to permit flow from said drum to said tubes when the pressure on the drum side of said valve exceeds the pressure on the tube inlet side of said valve.

References Cited by the Examiner UNITED STATES PATENTS Smith 122-196 Hedlund 122-35 Waterman 122-35 X F0111 122-406 X Dickey 122-451 Abendroth 122-448 Dickey 122-448 Dickey 122-448 Hartmann 122-489 X Wunsch 122-35 Hood 122-407 Koch 122-406 Great Britain.

EDWVARD I. MICHAEL, Primary Examiner.

PERCY L. PATRICK, FREDERICK L. MATTESON,

Examiners. 

1. IN A ONCE-THROUGH VAPOR GENERATING UNIT FOR SUPPLYING VAPOR TO A POINT OF USE, A THERMAL ENERGY SOURCE, A MULTIPLICITY OF LONG SMALL BORE TUBES ARRANGED IN PARALLEL FLOW RELATION TO GENERATE VAPOR FROM THE HEAT OF SAID SOURCE, MEANS SUPPLYING A VAPORIZABLE LIQUID TO THE INLETS OF SAID TUBES, A DRUM ARRANGED TO CONTAIN A BODY OF LIQUID BELOW AN UPPER VAPOR SPACE AND ARRANGED TO RECEIVE THE HEATED FLUID FROM SAID TUBES, MEANS FOR CONTINUOUSLY PASSING A SMALL QUANTITY OF THE HEATED FLUID DISCHARGING FROM SAID TUBES DIRECTLY TO THE LIQUID SPACE OF SAID DRUM AND FOR PASSING THE REMAINDER OF SAID HEATED FLUID TO THE VAPOR SPACE OF SAID DRUM, AND A CONTROL RESPONSIVE TO THE LIQUID LEVEL VARIATIONS IN SAID DRUM TO MAINTAIN A RATE OF FLOW OF SAID FLUID THROUGH SAID TUBES SUBSTANTIALLY EQUAL TO THE VAPOR DEMAND ON SAID UNIT. 